The third generation partnership project (3GPP) wideband code division multiple access (W-CDMA) system is outlined in the operational scenarios for universal mobile telecommunications system (UMTS) releases R99/R4 and R5. Release 5 of the UMTS frequency division duplex (FDD) and time division duplex (TDD) modes have incorporated a feature called high speed downlink packet access (HSDPA) for improving throughput, latency, and spectral efficiency in the downlink from the radio network to a wireless radio unit referred to as a user equipment (UE) or mobile stations. The principle of HSDPA is to schedule packet transmissions on the air interface to different UEs as a function of their instantaneous experienced radio and service conditions in a dynamic manner, (e.g., every 2 ms in FDD or every 10 ms in wideband TDD).
The key HSDPA functions in both FDD and TDD modes include: (i) fast retransmissions (Hybrid ARQ) of downlink packets received in error over the air interface, (ii) fast uplink notification of downlink packets received in error (Acknowledgements/Negative Acknowledgements), (iii) fast channel feedback in the uplink on the downlink channel state of a UE, and (iv) fat-pipe scheduling for efficiently servicing many users in the downlink. The dynamic HSDPA packet scheduler is located in the base station (referred to as the Node B in 3GPP) and operates substantially independently from the radio network controller (RNC).
The RNC in a UMTS network has responsibility for network control and radio resource management (RRM). The RNC performs tasks such as user admission control and interference management using dynamic channel allocation (DCA) algorithms to provide reliable system operation and good system efficiency. One measure of high efficiency is when a high level of overall throughput is achieved.
In an FDD system, the RNC allocates a certain number of spreading codes for the usage of HSDPA data channels (high speed-downlink shared channels (HS-DSCHs)) to each cell. Furthermore, in the FDD system, the HS-DSCH is transmitted over an HS transmission timing interval (TTI) length of 3 consecutive timeslots (3*0.66 ms=2 ms). The RNC communicates with the base station indicating what spreading codes can be used for HSDPA and subsequently passing control on when to send downlink packets using these codes to the base station. The RNC also notifies the UE by control signaling regarding which physical channels to listen for the HSDPA control channels, i.e., high speed shared control channels (HS-SCCHs), which in turn are used by the base station to dynamically notify UEs of the arrival of scheduled downlink packets on its HS-DSCH. Also, the same information is sent from the RNC to base station, such that the base station is informed regarding which HS-SCCH a UE is to be alerted when HSDPA data is to be sent to the UE. As mentioned already, the base station acts on an independent basis to determine, based on its HSDPA scheduler, when to transmit HSDPA data to a particular UE.
In a TDD system, the RNC allocates a certain number of timeslots (TSs) for the usage of HSDPA data channels (HS-DSCHs) to each cell. The RNC communicates to the base station that the TSs and spreading codes that can be used for HSDPA and subsequently passes control on when to send downlink packets using these TSs and codes to the base station. Similar procedures are then followed as described above for the FDD system.
In any CDMA system, efficient power management is important to keep interference low and to maximize the system capacity, i.e., the number of simultaneously supported users and overall data throughput for all cells in an area. For interference management, both FDD and TDD employ fast closed-loop power-control (PC) in the downlink for the dedicated channels. Furthermore, for the most common case of FDD and TDD conventional Release 99, 4, and 5 (R99, R4, and R5) dedicated channels (DCHs), that closed loop power control operates within RNC controlled power limits. Thus, a dynamic power range is pre-established at dedicated channel (DCH) setup and eventually adjusted during the life-time of the DCH by the RNC. The RNC signals the base station the PC dynamic range in the form of a maximum transmit (Tx) power not to be exceeded and a minimum Tx power to be maintained because the RNC must make complex decisions to enhance the system performance. For example, a UE requiring too much power, and thus frequently requiring the upper limit of the allowed dynamic range, creates excessive interference to other users in the system. The RNC may want to drop or to handover this UE's connection. For common channels of both TDD and FDD systems, control over the possible power settings is also important to ensure that adequate coverage and service are available.
The RNC allocates a maximum amount of downlink (DL) power as a fraction of the total available base station Tx DL power to maintain the relatively high-level of interference created by the HSDPA channels within reasonable limits. This is implemented by signaling over the RNC/base station interfaces (Iub) when configuring downlink channels in the base station. Otherwise, an HSDPA UE at a cell border could eventually be served by the base station at a high HSDPA data rate and create such a high level of interference that services in the neighboring cells would be heavily impacted and result in an unacceptable degradation of overall system capacity or service to non-HSDPA UEs. Another reason for the existence of such a control mechanism is that a certain amount of base station downlink Tx power needs to be reserved for non-HSDPA channels, such as pilot channels, common control channels, and non-HSDPA DCHs. The RNC set maximum HSDPA power fraction per cell in turn indirectly determines the maximum data rate with which any given UE can be serviced.
Related to power control is the variable quality of the radio channel or link from base station to UE. The detected radio channel or link quality depends on a number of factors including the transmit power level, the distance between the mobile terminal and a transmitting base station in the radio network, interference from other transmitting base stations and mobile terminals, path loss, shadowing, short term multi-path fading, etc. If the channel quality is good, the base station may modify the signal transmission parameters to increase the data transmission rate from the base station to the UE. On the other hand, if the channel quality is bad, the signal transmission parameters may need to be adjusted to lower the data transmission rate to ensure that the signal is reliably received.
The process of modifying one or more signal transmission parameters to compensate for channel quality variations is referred to as “link adaptation,” where “link” refers to the radio link between a base station and a mobile terminal. Link adaptation may be accomplished by changing the transmit power of the base station or effective bit rate over the link, e.g., increasing the transmit power level or decreasing the bit rate for data transmitted to mobile terminals with a bad channel quality. Link adaptation may also be accomplished by changing the type of modulation and the amount of channel coding applied to the data to be transmitted by the base station.
The UEs estimate channel quality by measuring the signal quality of pilot signals or other broadcast signals transmitted by nearby “candidates.” Based on the estimated channel qualities, each UE sends a channel quality index (CQI) report to the radio network that recommends a maximum data rate at which the UE can receive data from each candidate base station. The performance of a link adaptation scheme depends on the accuracy of the signal quality measurement made by the UE. It is especially important that the signal quality measurements do not over-estimate the future signal quality. In the case of over-estimation, the link adaptation will select transmission parameters that are not sufficiently robust for the actual channel condition. Ideally, the UE should accurately estimate a future radio channel condition at the time when the data packet transmission from the base station and use those current signal quality measurements as estimates of the radio channel condition when the future packet transmission occurs. Unfortunately, there are circumstances when such estimates are not accurate.
One such circumstance arises in systems where the HSDPA channels may use the remaining power of the total available power that is not used by other non-high speed (non-HS) channels. Therefore, when there are few non-HS users in the system, there can be a very large difference between the total power level in the cell before and after a HS-DSCH user starts receiving data in the cell. In that case, users in the cell will experience a large difference in the interference before and after a HS-DSCH user starts receiving data. Normally, a large increase in interference has the effect of reducing the obtained bit rate, since a more robust transport format is selected. The transport format selection is chosen to achieve a certain BLER (Block-Error Rate) based on the CQI reported by the UE and the available power. Hence, to accurately select the proper transport format, there should be a good match between reported CQI and channel quality when the actual transmission occurs. But in the scenario above, there may be a large mismatch between the reported CQI and the actual channel quality. Specifically, there may be a large mismatch in the initial choice of transport format because of a large difference in the total transmit power before and after a HS-DSCH user enters the system. In case of applications that generate data packets in many small bursts, such as TCP-based applications, this mismatch may occur frequently. For example, during the TCP “slow start” phase, which operates by controlling the rate at which new packets should be transmitted downlink based on the rate at which packet acknowledgments for previously transmitted packets are returned by the UE. This leads to a frequent series of retransmissions that result in a significant throughput reduction due to the “exponential backoff” of TCP, in which the retransmission timer is doubled for each retransmission. If an acknowledgement of a TCP segment is not received before the retransmission timer expires the TCP segment is retransmitted.
FIG. 1 shows a Cumulative Distribution Function (CDF) of the bit rates for 10 Mbyte web object downloads to a UE in a good receiving position. The CDF describes the probability that a variable takes on a particular value. The graph assumes only a single cell and no other UEs present in the cell. As shown, in approximately 50% of the downloads, the bit rate is below 1 Mbit/s, which is much lower than the maximum achievable bit rate of approximately 5.25 Mbit/s.
One way of dealing with this problem is to only permit a gradual increase in the power at which data is transmitted to the new HS-DSCH user. For example, the rate at which the total transmit power Ptx level in the cell can increase is limited a constant power increase factor. Gradual changes to the base station transmit power do not significantly affect the accuracy of the mobile terminal channel quality estimates and a decreased probability of low throughput is achieved because of fewer corrupted data packets and retransmissions. But a drawback with a constant gradual power adjustment approach is that for a small constant power increase factor, the maximum reachable throughput is reduced. This problem is illustrated in FIG. 2.
The graphs in FIG. 2 show CDFs using different constant gradual power adjustment factors F. The CDFs show that decreased probability of low bit rate is obtained at the cost of lower achievable bit rate. For example, for F=1.03, the CDF shows that at a probability of 0.1, i.e., in 10% of the downloads, a web object bit rate less than or equal to 4.6 Mbit/s is attained. In 90% of the downloads, a bit rate higher than 4.6 Mbit/s was attained. Although much improved, 4.6 Mbit/s is still lower than the maximum achievable bit rate of approximately 5.25 Mbit/s.
The present inventors recognized that a solution to this problem is to use a variable transmit power increase factor rather than a constant transmit power increase factor. In one non-limiting implementation, the power increase factor F is varied with the current total downlink transmitted power available for high speed downlink transmission. For example, when the available high speed shared channel power is large, the variable transmit power increase factor assumes a more conservative value, e.g., a smaller F value, at the beginning of the high speed downlink transmission, and then a more liberal value, e.g., a larger F value, later during the high speed downlink transmission.